The major objective of the proposed research is to test the hypothesis that orthodontic tooth movement and root resorption can be predicted by and will correlate with biochemical and histomorphometric measures of alveolar bone turnover. This will be tested in rats of two developmental stages (prepubertal and adult) with four levels of initial orthodontic force (0, 20, 40, and 60 grams). This research design will facilitate the measurement of bone turnover parameters in the alveolar bone adjacent to treated teeth under conditions where tooth movement and root resorption should vary considerably. About 2/3 of the population can be categorized as having a treatable malocclusion. Moreover, the demand for orthodontic services has risen steadily in recent decades such that orthodontics now constitutes a major category of dental treatments. Orthodontic tooth movement remains one of the major means by which malocclusions are treated and represents a rate- limiting step in treatment time. Root resorption is cited as one of the major unwanted sequelae to orthodontic tooth movement. There is considerable individual variation in orthodontic tooth movement and root resorption and the degree to which biological parameters contribute to this variation is not well understood. The long-term goal of this work is to identify the significant bone turnover parameters in orthodontic tooth movement and root resorption and to study their impact on individual variation in the clinical context. Using the rat model we will alter one biological (developmental stage) and one biomechanical (force magnitude) parameter, to create significant changes in the rates of tooth movement and root resorption. These will then be followed as a function of time post-appliance activation. Alveolar bone turnover will be monitored biochemically and histomorphometrically. The biochemical parameter of bone formation will be tissue alkaline phosphatase and osteocalcin; for the resorption, they will be acid phosphatase and tartrate-resistant acid phosphatase. Histomorphometric measures of bone turnover will include measurements of several specific histological features characteristic of bone formation and resorption utilizing routine undecalcified histological preparations and tetracycline- generated vital bone stains.